Moment of inertia is the sum of the products formed by multiplying the mass of each part of an assembly by the square of its distance from a specified axis plus the moment of inertia of each part about its own center of mass. It is also sometimes referenced as rotational inertia. In spherical objects, such as a golf ball, a low moment of inertia means that a larger portion of its mass is concentrated in the center. In turn, a high moment of inertia means that more of its mass is concentrated towards the outer cover or periphery of the sphere or ball.
Moment of inertia (“MOI”) affects the playability of a golf ball in many ways. For example, moment of inertia affects the amount of spin produced when the golf ball is struck with a wood or iron. This may result in a desirable characteristic (i.e. high spin for ball placement, low spin for enhanced distance, etc.) or an undesirable characteristic (i.e. hooking or slicing, etc.) depending upon the skill level of the golfer, the type of club used, etc. Moment of inertia also affects the overall trajectory of a ball and thus, often the overall distance the ball will travel. Also, moment of inertia affects the short game, including lofting, pitching, chipping, and putting.
For some aspects of the golf game, it is desirable to have a golf ball that exhibits a relatively high moment of rotational inertia, for example in which the mass of the ball located near the outer periphery of the ball is greater than the mass of the ball located near the center of the ball. A golf ball exhibiting a high moment of inertia generally has a reduced rate of spin, including reduced side spin, so that such a ball may be desired for certain shots requiring distance. A low spin ball also produces less side spin, thus reducing the amount of hooking and slicing.
Although a golf ball exhibiting a relatively high moment of inertia has certain desirable properties at different club head speeds and with different lofted clubs, it may also possess undesirable characteristics. Furthermore, such a ball may lack the necessary feel and roll characteristics for the short game, particularly putting.
Therefore, for certain aspects of the golf game, it would be desirable for a golf ball to exhibit a relatively low moment of inertia, where the mass of the ball near the center of the ball is greater than the mass of the ball near the outer portion of the ball. As noted, in some applications, a golf ball exhibiting a relatively low moment of inertia is desirable, such as for the short game where high spin allows a skilled golfer to more easily position his/her ball on the green, etc. In turn, it is also desirable in certain situations for a golf ball to exhibit a relatively high moment of inertia, such as for the long game where enhanced distance is desirable.
Currently, golf balls having a relatively low or high fixed moment of inertia are commercially available or are known in the art. For example, U.S. Pat. No. 5,026,067 teaches a golf ball having a cover or intermediate layer with a specific gravity greater than the center, giving the golf ball a relatively high moment of inertia. Also, U.S. Pat. No. 6,010,912 teaches a golf ball having a low specific gravity core and a high specific gravity layer surrounding the core so that the golf ball has a relatively high moment of inertia. Conversely, U.S. Pat. No. 6,180,722 teaches a golf ball having a specific gravity near the center of the ball greater than the layer surrounding the center so that such a ball has a relatively low moment of inertia.
Players, depending upon their skill and preferences relating to the features of a golf ball, may choose a golf ball having a relatively high moment of inertia in order to increase distance and/or reduce the amount of slicing or hooking when driving, or a golf ball having a relatively low moment of inertia for improved feel, placement, etc., near or on the green. Unfortunately, a golfer choosing either a high or low moment of inertia golf ball in order to promote certain aspects of the game risks suffering deficiencies in other aspects of the game.
In the past, golf and/or game balls have been formed where the movement of inertia of the ball randomly varies. U.S. Pat. No. 1,120,757 discloses a game ball having a spherical ball that randomly moves within the chambers of the game ball when the game ball rotates. The springs described in that patent are used to ensure that the spherical ball does not maintain a position near the outer periphery of the game ball. That is, the springs cause the spherical ball to rebound from the outer periphery of the game ball toward the center of the game ball, thereby preventing the spherical ball from remaining in one chamber while the game ball is in action. The design of the game ball in the '757 patent causes the center of gravity, also known as the center of mass, of the game ball to randomly vary as the game ball rotates as a result of the random movement of the spherical ball within the interior of the game ball. This design causes peculiar gyration and movement of the game ball along paths that are impossible to determine with any degree of accuracy.
U.S. Pat. Nos. 728,311; 737,032; and 2,859,968 are directed to various balls that have a hollow portion in the center of the ball and one or more smaller balls within the hollow portion. When the ball rotates, the smaller ball or balls in the hollow portion freely moves in an uncontrolled fashion within the ball. None of the balls disclosed in these patents has a controlled variable moment of inertia which would maximize the playability and feel desired in a golf ball.
Accordingly, it would be useful to develop a golf ball having a controlled variable moment of inertia such that the golf ball exhibits low moment of inertia properties that are desirable during short distance play and also exhibits high moment of inertia properties that are desirable during longer distance play. In particular, it would be desirable to provide a golf ball having a moment of rotational inertia that may be selectively varied. It would also be desirable to provide a golf ball that has excellent durability.